It is well known that different constituents can be measured in solutions, particularly in milk, by infrared transmission spectroscopy apparatus. In liquids, where particle sizes are of the same magnitude or larger then about ⅓ of the electromagnetic wavelength, light scattering attenuates the transmission. The milkfat in milk is comprised of fat globules with an average diameter of app. 3–4 μm and with largest globules of about 10 μm. The triglycerides have four major absorbance wavebands in the mid infrared regions between 2.5–10 μm, namely at about 3.5, 5.7, 6.8 and 8.6 μm, which are caused by C—H stretching vibrations, C═O stretching vibrations, C—H bending vibrations and C—H rocking/C—C stretching vibrations, respectively. The two first wavebands are exclusively used for the determination of fat in milk, due to their intense absorbance as well as high accuracy (as compared to standard chemical determination e.g. Rose-Gottlieb) in determining the fat content in milk. However, these two wavebands have shown to be afflicted by anomalous dispersion phenomena, giving a large variation in the refractive index across the wavebands. The combination of light-scattering and anomalous dispersion give an increasing distortion in the shape of the fat absorbance peaks as the fat globules size increases. This effect, the so called Christiansen effect, has lead to that all the presently preferred apparatuses are provided with built-in homogenizers. Also known apparatus for infrared determinations are using double wavelength single cuvette system as mentioned in U.S. Pat. No. 4,247,773. In practice, even apparatus having the ability to use full-spectrum, are using traditional wavebands with at least one reference waveband for determination of the major component in milk (fat, protein, lactose), and the milk to be analyzed is homogenized with a built-in homogenizer prior to the determination. As long as the homogenizer is working properly the accuracy of the fat determinations is very good. The absorbance bands above 6 μm are mainly influenced by light scattering and the effect is decreasing as the wavelength increases. Many attempts have been proposed in order to enable determination without homogenization. In the patent application WO 92/17767 a method by using a waveband of approx. 1160–1190 cm−1 (8.64 μm) is proposed as a method for measuring the fat content in e.g. milk. However, at this waveband the fat has a weak absorbance and the accuracy (as compared to standard chemical determination) is not nearly as good as what is obtained with the wavebands below 6 μm, in the region of 3.5 and 5.7 μm, The effect of light-scattering can be minimized by using a reference near the fat peak in the region of 8.6–8.7 μm as the scattering is nearly of the same magnitude in both the sample waveband and the reference waveband region. The main reason for using the proposed waveband as in WO 92/17767, was to minimize the interference from other constituents of the emulsion. The proposed waveband has also been disclosed as a total solid or moisture waveband in another patent application (EP 0 404 207 A2, U.S. Pat. No. 4,825,076), which also indicate that the proposed waveband also have high correlation to dry-matter in the milk. In WO 93/06460 an infrared attenuation measuring system is disclosed, and among other things a fat determination method, which is substantially independent on the degree of homogenization.
The need for efficient built-in homogenizers for the infrared determinations causes problems of maintaining the efficiency of the homogenization. The homogenizers are built up with moving parts and are relatively large in size, and therefore it is difficult to develop small in-line infrared sensors. A recent patent application (WO 01/04612) discloses a method and apparatus for measuring the fat content in unhomogenized milk and the variation of light-scattering is compensated by means of at least two statistical parameters. The proposed method is using a single waveband for each constituent and the measurement is repeated for a number of aliquots of a sample. A method that can perform determinations without any homogenization (reduction in the fat globule size) and still having a high accuracy means considerable simplification and improvements in development of apparatus. The possibility of making small, simple and robust apparatus with high accuracy, implies that cow-side and in-line apparatus can be produced. This is also an urgent need for developing countries since existing apparatus is expensive and/or developed for large-scale laboratories.